Electro-mechanical transfer switches are electrically operated and mechanically held double throw switches used to transfer an electrical load between two power sources. For example, a transfer switch might be used to transfer a load from a “normal” power source such as standard 120 volt or 240 volt AC service, to an “auxiliary” power source such as a generator. The transfer can occur in response to detecting a malfunction of the normal power source or because the normal power source is being serviced, for example. These transfer switches generally include a switching mechanism that is magnetically coupled to a solenoid. When a control current is provided to the solenoid, the switching mechanism toggles its position to transfer the load from the normal power source to the auxiliary power source.
Often, the solenoid will be operated via a normally open switch connection to the normal power source and/or to the auxiliary power source, depending on the situation. Typically, both types of power sources will produce waveforms that exhibit non-idealities such as amplitude variation or frequency variation. Changes in the voltage amplitude across the solenoid or the current amplitude through the solenoid cause undesirable changes in the time it takes for the transfer switch to transfer the load between power sources. For example, the transfer time generally increases in response to a decrease in solenoid current or voltage amplitude. Changes in ambient temperature can also cause the transfer time to vary undesirably. For example, an increase in ambient temperature will generally cause reduced current through the solenoid, and thus, an increased transfer time.